Intervertebral disc prosthetics must comply with several requirements, often contradicting each other. These requirements are derived from studies on natural intervertebral discs. The prosthetic artificial disc should provide angular mobilities with specified ranges and stiffness values (two bending mobilities, fore-and-aft and side-to-side, and torsional around the axis of the spine), while accommodating large and varying axial (vertical) loading; it is desirable that the stiffness values are not significantly influenced by the changing vertical loads. Dummies are mock-ups of human bodies designed to simulate behavior of human bodies in extreme circumstances, e.g. crash dummies. Dummies are not, usually, equipped with all individual vertebras simulating the human spinal column but have structural elements rather crudely simulating the human spine. These structural elements will also be called “vertebras” in this Specification. Maintaining the structural characteristics of artificial intervertebral discs in close similarity to the natural discs is very important in order to adequately simulate behavior of the human bodies in the course of the dummy-based experiments.
There are various proposed designs of artificial intervertebral discs attempting to simulate structural characteristics, especially stiffness values and ranges of motion of the natural discs. The most widely used designs comprise spherical joints generated by a concave spherical socket engaged with a fitting convex spherical protrusion. Both surfaces are usually made from a low friction plastic capable of sliding without lubrication. While providing mobility in various angular directions and capable of accommodating axial loads, these designs have several shortcomings.
These artificial discs do not have elastic characteristics resident in the discs of the natural spinal column in any of the three angular directions. The natural elastic resistances to the motions approximately proportional to the deformation angle are replaced in these prosthetic or artificial discs by frictional resistances, practically independent on the motion magnitude.
Secondly, the motion resistances in all three directions (the friction forces) are increasing with the increasing axial force in the spinal column (which varies in the wide range). This also results in some unnatural feelings, since the elastic resistance forces in the natural spinal column are not significantly dependent on the axial force.
Another shortcoming of the state-of-the-art prosthetics is an unavoidable difference between static and dynamic friction coefficients in spherical joint. This makes the motion resistance different in the beginning of the movement and in the process of movement, since the static friction coefficient is greater than the dynamic friction coefficient.
Yet another shortcoming, which is also a result of frictional interaction in the spherical joint, is inevitable wear of the sliding connection. The wear is enhanced by sometimes high axial pressures in the spinal column which are too high for sliding plastic contacts. The wear process creates worn-out particles which are contaminating the area around the disc and may increase the friction forces if accumulated in the sliding connection.
The subject invention eliminates the listed shortcomings.